This invention relates to waveguide structures embedded in a substrate, and more particularly, to planar and channel waveguide structures such as those employed in arrayed waveguide gratings (AWGs), specifically to waveguide structures manufactured by ion diffusion into a substrate to change locally the refractive index of the substrate.
Planar waveguides and channel waveguides, usually made on a silicon substrate by deposition and etching or other semiconductor techniques, are commonly known in the art.
Ion exchange and other ion diffusion techniques have been receiving increased attention as methods to produce channel optical waveguides in glass and other materials. The importance of the resulting channel glass waveguides stems from their compatibility with optical fibers, potentially low cost, low propagation losses and other factors. However, while the deposition/etching approach produces a relatively well-defined refractive index profile in the core layer, it is virtually impossible to produce equally well-defined refractive index profile in glass substrate by ion diffusion techniques.
A typical diffusion process is exemplified by an ion-exchange process wherein a piece of glass or another suitable material (a wafer) is contacted (e.g. immersed) with a melt containing desired ions. The ions of the melt, e.g. Ti+, Cs+, K+, Li+, Ag+, Rb+ ions, chosen to have a higher polarizability than the ones of the wafer, are exchanged with ions from the wafer, typically Na+. A review of the ion-exchange techniques is given by Ramaswamy et al., J. of Lightwave Technology, Vol. 6, No. 6, p.p. 984-1002, June 1998. Generally, the techniques include depositing a metallic mask, with slots made e.g. by photolithography, on the glass substrate, contacting the substrate with melt containing selected cations, and, once surface waveguide(s) is produced by ion exchange and diffusion, optionally an application of electromagnetic field to force the cations below the surface to produce xe2x80x9cburiedxe2x80x9d waveguides.
The refractive index is locally increased in the substrate because of three factors: local change of the glass density, higher polarizability of the locally exchanged ions, and local stresses.
It is obvious, as can be seen in FIG. 1, that only a small fraction of the wafer surface is processed when making a conventional channel waveguide structure. From the viewpoint of the material, the waveguides can be considered as local defects in the wafer material composition. The physical properties of the wafer and the exchanged ions (birefringence, heat conductivity, heat capacity, diffusion constant and mobility) are locally modified in the areas where the ions were exchanged, i.e. where the waveguides were created. The result is that the optical properties of the waveguides (insertion loss) may not be uniform over the wafer (in one case the spatial change is between cladding and core region, in the other case, the spatial change is between core to core). Referring to FIG. 1, showing 6 couplers, the waveguides in the middle (2-5) all xe2x80x9cseexe2x80x9d the same environment (say, environment A). The two outer waveguides, on the other hand, both xe2x80x9cseexe2x80x9d a same environment B that is different than the A. Environment A consists of other waveguides, whereas environment B consists of waveguides on one side and semi-infinite cladding on the other side. This may result in different insertion losses and higher PDL (higher birefringence due to uneven stress increases the PDL) for the two groups of waveguides (four inner waveguides on ones side, and two outer waveguides on the other side).
It is an object of the invention to xe2x80x9cequalizexe2x80x9d environment A and environment B which would both tend to be infinite cores.
Another object of the invention will be evident from the following discussion comparing a slab waveguide and a channel waveguide. The ion density is bigger in one case (slab waveguide) than in the other. Since the diffusion constant and the mobility of the exchanged ions are concentration dependent, the diffusion equation will write differently in the two regions, resulting in different refractive index distributions. Since the heat conductivity and capacity are also concentration dependent, the heat diffusion equation will also write differently in the two regions, resulting in different temperature distributions (although both the slab and the channel region are poured in the melt having a uniform temperature, their thermal behaviour is different), thus in different refractive index distribution. In particular, under the same process conditions (same temperature, same E-field, same exchange time . . . ), a slab waveguide is buried deeper than its channel counterpart, due to the combined effects. It should be noted that the heat conductivity and heat capacity are wafer properties, whereas the diffusion constant and mobility are ion properties in a given wafer; the same ions have different diffusion constants and mobilities in different glasses, and different ions have different diffusion constants and mobilities when embedded in one common glass.
The presence of the local defects corresponding to the core region of the glass waveguide gives rise to undesired mechanical stress in the wafer. As a secondary issue, the use of a metallic mask on one surface wafer can also create stresses due to different thermal expansion coefficient of typical mask metals compared with that of glass, when the wafer with the mask are subjected to elevated temperatures of the melt.
It follows that it is desired to maintain as high as possible uniformity of the glass wafer waveguide. However, this is contradictory with the need to produce local xe2x80x9cdefectsxe2x80x9d in the wafer, the defects representing the core region.
It is desirable to maintain the optical properties of the waveguides as uniform as possible over the wafer. This is virtually equivalent to maintaining the glass material properties (e.g. composition, birefringence, heat conductivity, heat capacity) as uniform as possible over the wafer, which, in turn, is equivalent to maintaining the ion density as uniform as possible over the wafer.
U.S. Pat. No. 5,940,555 to Inaba et al. proposes an optical multiplexer/demultiplexer structure made on a quartz substrate by etching, the structure having a number of artificial channel waveguides adjacent an arrayed waveguide grating. The presence of the artificial waveguides is meant to reduce a variation and fluctuation in (etched) core width and to improve crosstalk characteristics of the grating.
In accordance with the present invention, there is provided a waveguide structure comprising:
a planar substrate,
a waveguiding core region on the substrate comprising additional ions distributed over the core region at a first density range,
a boundary region contiguous to and surrounding the core region, substantially devoid of additional ions and having a refractive index, the boundary region forming a cladding about the core, wherein the core region has a refractive index range, caused by and commensurate with the first ion density range, generally higher than the refractive index of the boundary region, and an outer region surrounding the boundary region and comprising the additional ions distributed over the outer region at a density range comparable with the first density range.
The term xe2x80x9cadditional ionsxe2x80x9d is used herein to denote ions from an outside source that have either been exchanged from the outside source with original ions of the substrate, or added (e.g. implanted) from an outside source without replacing original ions of the substrate. The additional ions should not be of the type already present in the substrate in any significant quantities.
In accordance with another aspect of the invention, there is provided a method of fabricating an optical waveguide circuit on a substrate, e.g. a chip or wafer, the method comprising the steps of
supplying additional ions at a first density range into at least one waveguiding core region on the substrate, and
supplying the additional ions at a second density range, comparable to the first density range, into a portion of the substrate isolated from the core region by a boundary region substantially devoid of additional ions.
The additional ions may be supplied by ion exchange or another ion diffusion process in which ions are diffused into and over the substrate.
The ions may be supplied into the respective regions of the substrate using a mask having at least one substantially ion-transparent region and an ion-opaque region, the at least one ion-transparent region of a shape corresponding to at least one waveguiding core, and the opaque region of a shape corresponding to the boundary region directly adjacent to the core.